1. Field of Invention
The present invention relates to a sheet metal container and more particularly to an improved container having sheet metal members with different electrode potentials isolated in order to resist galvanic corrosion.
2. Description of the Prior Art
A common starting workpiece for a formed sheet metal container is a blank severed from flat or coiled stock sheet metal. For economic reasons, it is common practice to protectively coat both sides of the flat stock sheet metal. This insures that both surfaces of the blanks will be coated after severance. An inherent deficiency in this practice is that the edges of the blank are not protectively coated.
Containers for foods and beverages must be protectively coated for a variety of reasons among which are considerations of taste, appearance and odor. Also, since containers may be constructed of metals having different electrode potentials, and since the contents could serve as an electrolyte it is often necessary to isolate the metals even at the exposed edges in order to resist galvanic corrosion. Examples of very corrosive foodstuffs include tomatoes, citrus juices, cherries and prunes.
Galvanic corrosion, or electrochemical corrosion, occurs when current flows between cathodic and anodic areas on metallic surfaces in a galvanic cell of two conductors having different electrode potentials in an electrolyte. Electrode potential refers to the ability of a conductor to corrode in a galvanic cell and has been used as a metallurgical research tool by measuring the potential against a 0.1 N calomel reference electrode in a standard NaCl--H.sub.2 O.sub.2 solution of 53 gram per liter NaCl + 3 gram per liter H.sub.2 O.sub.2 at 25.degree. C. The electrode potentials of some metals and alloys are as follows:
______________________________________ Metal Potential ______________________________________ Magnesium -1.73 Zinc -1.10 Mild Steel -0.58 Lead -0.55 Tin -0.49 Copper -0.20 Stainless Steel -0.09 (Series 300 Type 430) Aluminum Solid Solutions or Constituents Mg.sub.2 Al.sub.3 -1.24 99.95 Al -0.85 CuAl.sub.2 -0.73 FeAl.sub.3 -0.56 ______________________________________
In a galvanic cell, i.e. a cell in which chemical change is the source of electrical energy, the anode is more electrically reactive than the cathode. This difference in potential causes current to flow from the cathode to the anode through the metal, and from the anode to the cathode through the electrolyte to complete the circuit. Where the current enters the electrolyte, metal ions go into solution causing corrosion of the anode.
In the manufacture of cans, it would be common practice to join a tin plated steel body to an easy-open aluminum can end if it were not for galvanic corrosion. If these metals are not isolated from each other, the aluminum will tend to act as a sacrificial anode in a galvanic cell and may be destroyed. Methods of preventing anodic action or galvanic corrosion at the seam have been disclosed in the prior art, including for example U.S. Pat. No. 3,439,641. The present solutions include inserting a plastisol sealant in the seam and dip coating the flanges in a sealant bath prior to seaming in order to isolate the exposed metallic edges from each other. Individually coating the edges of each container blank is sometimes effective, however, it is a time consuming and thus expensive operation.
The prior art pertaining to sheet metal containers is replete with examples of seams and methods of forming seams. Some of the prior art patents disclose lapping, bending and folding of the terminal end of a sheet metal flange prior to rolling a seam, but none of these methods appear to provide resistance to galvanic corrosion. For example, O'Brien U.S. Pat. No. 2,362,846 discloses bending the flange back upon itself prior to rolling the seam to create a bulge in the seam in order to facilitate cutting the outer wall. Also, Bloedorn U.S. Pat. No. 2,382,378 discloses a hem at the end of the body of a pry-off container which provides a lock shoulder in order to engage a bead in the container cover in metal-to-metal contact. Pearson U.S. Pat. No. 2,424,188 pertains to forming a sideseam in a sheet metal can body by having one side edge portion lapped in order to avoid the exposure of raw metal on the inner side of the can.
Accordingly, an economical and effective improvement is required to assure that metals having different electrode potential are isolated after they are seamed together in order that galvanic corrosion is resisted.